MMR and MSI Testing in Patients Being Considered for Checkpoint Inhibitor Therapy

MMR and MSI Testing in Patients Being Considered for Checkpoint Inhibitor Therapy

Background

The US Food and Drug Administration (FDA) recently granted accelerated approval to the immune checkpoint inhibitor pembrolizumab for patients with any advanced tumor malignancy that is microsatellite instability (MSI)-high or mismatch repair (MMR) deficient. The FDA did not specify which assay should be used to assess for mismatch repair or microsatellite instability.

It is critical that practicing pathologists make informed decisions about introduction of new biomarkers into their practice. Many of new biomarkers have associated high costs both in the form of capital and reagent expenditures or as send out tests. It may be difficult to quickly access unbiased information and pathologists may find themselves unduly influenced by marketing campaigns or by focused and possibly impractical demands from other clinicians.

The upcoming recommendations will impact testing for patients with colorectal, endometrial, gastroesophageal and small bowel, and other types of cancers. In addition, they provide guidance on the role of tumor mutational burden in MMR testing and the evaluation of Lynch Syndrome.

Scope

The primary goal of this guideline is to develop evidence-based recommendations for the testing of MSI and/or MMR in patients with advanced solid tumor malignancies being considered for checkpoint inhibitor therapy.

Overarching Questions

Which test modality best predicts DNA MMR?

Do MMR by immunohistochemistry, polymerase chain reaction, or by next generation sequencing results predict improved clinical outcomes in patients treated with checkpoint inhibitors?